1. Field of the Invention
The present invention relates to a pulse arc welding apparatus for arc-welding by feeding a consumable electrode to a workpiece (a part to be welded) with a constant rate, and applying a welding current between the consumable electrode and the workpiece.
2. Description of the Related Art
A pulse arc welding method for welding by feeding a consumable electrode with a constant rate implements a welding by superimposing a pulse current equal to or more than a critical current value which is called a transition current, on a base current of maintaining an arc.
In the pulse arc welding method, such welding cases of combining workpieces of different plate thicknesses and of combining workpieces of different materials are increasing. Also, in the welding apparatus, automation of welding by a robot or an automatic-machine is greatly increasing, and thus is needed a technology of controlling the welding conditions (such as current, voltage, travel speed, etc.), in a preset or in a real-time, in response to a change of a joint of the workpiece, by a combination of vision sensors and the like.
Recently, in a welding procedure of a welding system in which the robot and various sensors are combined, the procedure of welding by changing the welding conditions to equal to or more than that of the conventional welding procedure, according to the changes of plate thicknesses, materials and gaps, in one of successive welding joints, is increasing. Accordingly, it is necessary to stabilize promptly to an arc length which is suited to the conditions when changing the set values of the welding conditions, and in addition, for a wire stick-out length which changes relative to the plate thickness and the joint shape, the necessity of keeping it to a proper arc length is increased. That is, control of the arc length must be fast.
The applicant of the present application has already proposed the technology to greatly increase the arc length control in the pulse arc welding (Japanese Unexamined Patent Publication No. 5-23850). In this prior art, as shown in FIGS. 1 and 3 thereof, in order to ensure a stable weld of one pulse-one droplet transfer, setting an upper limit value and a lower limit value in a pulse current with a constant voltage characteristics are set. When the arc length has been changed suddenly, in the case of an extremely short or long arc length as to reach the upper limit value or the lower limit value, based on a different signal between the set value and the detected value of the pulse current, in addition to a control of increasing/decreasing a pulse frequency f, increasing/decreasing a predefined value .DELTA.f in the pulse frequency f, and increasing a variational range of an average current value, greatly control of the arc is greatly increased.
However, in the modern welding procedure, a large fluctuation in the welding condition set value or in the wire stick-out length can not be compensated by only increasing/decreasing the predefined value .DELTA.f in the pulse frequency, as described in the above mentioned pre-filed patent application, and it will take too much time until a proper arc length can be achieved. For this reason, in a case where the arc is long, a welding bead will be dented, and a concentration of the arc will vanish, and thus there exist weak points such that the beads are broken off, and the deflection beads occur. Also, in a case where the arc length is short, the welding beads will also be conveyed, and the arc shortage will occur, or the burn-through will occur for thin workpieces. In either case, it has been very difficult to obtain a good welding result.